At least 276 killed in Somalia's deadliest attack on record

The death toll has risen to 276, with more than 300 people wounded, after a truck bombing in Somalia that the U.S. government condemned Sunday as a "cowardly" attack.

"Such cowardly attacks reinvigorate the commitment of the United States to assist our Somali and African Union partners to combat the scourge of terrorism," the U.S. mission to Somalia said in a statement.

The U.S. military this year has stepped up drone strikes and other efforts this year against the al-Qaida-linked al-Shabab, which is based in Somalia and often targets Mogadishu, the capital.

None of the roughly 400 U.S. troops in Somalia were hurt in the attack, a spokesman for the U.S. Africa Command told Fox News. 

The International Committee of the Red Cross said four volunteers with the Somali Red Crescent Society were among the dead.

A statement Sunday said "this figure may rise as there are a number of volunteers still missing."

Hospitals in Mogadishu struggled Sunday to cope with the high number of deaths and injuries, security and medical sources said.

Ambulance sirens echoed across the city a day after the blast as bewildered families wandered in the rubble of buildings, looking for missing relatives.

President Mohamed Abdullahi Mohamed declared three days of mourning and joined thousands of people who responded to a desperate plea by hospitals to donate blood for the wounded victims. "I am appealing all Somali people to come forward and donate," he said.

Many victims died at hospitals from their wounds, said Police Capt. Mohamed Hussein.

Abshir Abdi Ahmed, a senator, announced the number injured, citing doctors at hospitals he has visited in Mogadishu.

Somalia's government has blamed the Al Qaeda-linked al-Shabab extremist group for the attack it called a "national disaster." However, al-Shabab, which often targets high-profile areas of the capital with bombings, had yet to comment.

"They don't care about the lives of Somali people, mothers, fathers and children," Prime Minister Hassan Ali Khaire said. "They have targeted the most populated area in Mogadishu, killing only civilians."

Somalia's information minister, Abdirahman Omar, said the blast was the largest the city had ever seen. "It's a sad day. This how merciless and brutal they are, and we have to unite against them," he said, speaking to the state-run radio station.

Overnight, rescue workers with torch lights searched for any survivors trapped under the rubble of the Safari Hotel, which was largely destroyed. The hotel is close to Somalia's foreign ministry. The blast blew off metal gates and blast walls erected outside the hotel.

Fox News' Lucas Tomlinson and The Associated Press contributed to this story.